The fluid catalyst cracking process (hereinafter FCC) has been extensively relied upon for the conversion of starting materials, such as vacuum gas oils and other relatively heavy oils, into lighter and more valuable products. In an FCC reaction zone, the starting material, whether it be vacuum gas oil or another oil, is contacted with a finely particulated, solid catalytic material that behaves as a fluid when mixed with a gas or vapor. This catalytic material possesses the ability to catalyze the cracking reaction. During the cracking reaction, coke is deposited on the surface of the catalyst as a by-product of the cracking reaction. Coke is comprised of hydrogen, carbon and other material such as sulfur, and it interferes with the catalytic activity of FCC catalysts. Facilities for the removal of coke from FCC catalyst, so-called regeneration facilities or regenerators, are ordinarily provided within an FCC unit. Typically, coke-contaminated catalyst enters the regenerator and is contacted with an oxygen containing gas at conditions such that the coke is oxidized and a considerable amount of heat is released. A portion of this heat escapes the regenerator with the flue gas, comprised of excess regeneration gas and the gaseous products of coke oxidation. The balance of the heat leaves the regenerator with the regenerated, or relatively coke free, catalyst. The high heat evolved in the regeneration process creates high temperatures in the regenerator. In order to withstand the high temperatures, the large regeneration vessel has an internal concrete-like refractory lining that insulates the metal shell from the high regenerator temperatures and erosion of the abrasive catalyst.
The fluidized catalyst is continuously circulated from the reaction zone to the regeneration zone and then again to the reaction zone. The fluid catalyst, as well as providing catalytic action, acts as a vehicle for the transfer of heat from zone to zone. Catalyst exiting the reaction zone is referred to as being “spent”, that is partially deactivated by the deposition of coke upon the catalyst. Catalyst from which coke has been substantially removed is referred to as “regenerated catalyst”.
The rate of conversion of the feedstock within the reaction zone is controlled by regulation of the temperature, activity of catalyst and quantity of catalyst therein, as measured by catalyst to oil ratio. The most common method of regulating the reaction temperature is by regulating the rate of circulation of catalyst from the regeneration zone to the reaction zone, which simultaneously regulates the catalyst/oil ratio. That is to say, if it is desired to increase the conversion rate, an increase in the rate of flow of circulating fluid catalyst from the regenerator to the reactor is effected. Inasmuch as the temperature within the regeneration zone under normal operations is considerably higher than the temperature within the reaction zone, this increase in influx of catalyst from the hotter regeneration zone to the cooler reaction zone effects an increase in reaction zone temperature.
An increasing number of FCC units are processing heavier feedstocks that produce more coke, which results in excessive combustion temperature that may exceed the metallurgical limits of the vessel and in high catalyst temperature that reduces the catalyst circulation rate to the reaction section and thus limits operational flexibility. The term external catalyst cooler generally refers to a shell and tube heat exchanger that circulates catalyst from the regenerator on the shell side of the exchanger and saturated steam or water on the tube side of the exchanger. Gas nozzles or aeration piping keeps the catalyst in fluidized state so that the fluidized catalyst heat exchange indirectly with the water or steam and results colder catalyst that circulates through the cooler and provides a source of relatively lower temperature catalyst for recirculation to the regenerator or return to the FCC reaction zone. By lowering the temperature of the catalyst, independent of the coke combustion, the cooler allows the FCC unit to fully or partially combust coke without excessive temperatures or to control the catalyst circulation rate independent of the riser temperature.
Locating the heat exchanger tubes outside of the regenerator in an external cooler permits isolation from a majority of the catalyst inventory in the event of a tube rupture or other operational problems. The external location of the cooler relative to the regenerator requires a circulation of catalyst between the cooler and the regenerator. Normally the circulating catalyst enters or exits the cooler from a large open volume of the regenerator. Gas nozzles or aeration piping keep the catalyst in a fluidized state so that it can circulate through the cooler. The cooler can operate in a flow through mode where hot catalyst enters one end of the cooler and leaves through from an opposite end of the cooler or in a backmix mode where the catalyst enters and leaves through the opening without any net flow. Gas injection into the catalyst is not only useful to provide the necessary interchange of catalyst through the cooler but also provides the flexibility to vary the heat transfer rate between catalyst and water.
Accordingly, it is desirable to provide apparatuses and methods for cooling catalyst. In addition, it is desirable to provided apparatuses and methods for cooling catalyst with a heat exchange tube protected with a hard surfacing material to prevent impingement thereof. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.